The present invention relates to erasers compositions, and more specifically to thermoplastic erasers.
Natural rubber erasers and plastic erasers are generally known. The natural rubber eraser utilizes the natural rubber as a matrix along with factice and a filler. These erasers are often finished by being lightly vulcanized with sulfur. The plastic eraser is generally an emulsion polymerized type of vinyl chloride resin matrix which is mixed with a filler and finished by steam treatment or the like.
Many plastic-based erasers have been developed. One example of the prior art erasers is disclosed in U.S. Pat. No. 4,918,130 which discloses an eraser comprised of a styrene thermoplastic elastomer matrix containing a polyolefin and a hydrocarbon resin. The eraser disclosed in this patent includes the use of certain additives where needed. One such additive disclosed is calcium carbonate, added as a filler. Also disclosed are the use of softeners such as process oils or polybutylene. The eraser of this patent is specifically concerned with erasing plastic crayon marks.
U.S. Pat. No. 4,374,225 discloses a method for manufacturing an eraser containing a low crystallizable polyolefin thermoplastic elastomer and, what the patent describes as, normal synthetic rubbers having no hard segment for crosslinking. Examples of these elastomers, according to the patent, include chlorinated polyethylene, ethylene-propylene termpolymer, ethylene ethyl acrylate, and styrene-butadiene rubber. The eraser disclosed in this patent is asserted to exhibit physical properties similar to a vulcanized rubber but without a vulcanizing operation and subsequent crosslinking. The eraser includes a factice, a filler, and a pigment.
U.S. Pat. No. 4,796,328 discloses an eraser having an integral structure of many eraser pieces, with the different pieces each having different hardnesses and abrasiveness. According to the disclosure, the soft eraser pieces within the eraser deform in conformity with any minute irregularities on the paper surface which leads to close contact of the entire eraser with the paper. In such a case, the abrasive material is forcibly pressed against the paper surface by the harder eraser pieces to scrape out the ink bonded on the paper surface.
Generally, the prior art erasers are concerned with frictional scraping of the mark to be erased. Such a focus lead to the development of erasers, the use of which often results in the abrasion of the surface (typically paper) and concordant unwanted destruction or tearing of the surface. Thus, a need exists for an eraser, which is effective at removing a mark from a surface but which does not tend to abrade or otherwise damage the surface.
The present invention provides an eraser in order to solve the problems with the prior art. It has been discovered that a superior eraser comprises 22-41 wt % styrene-ethylene/butylene-styrene block copolymer, 5-15 wt % ethylene/propylene copolymer, 30-50 wt % calcium carbonate, 10-25 wt % factice, and 0.1-7 wt % pumice. The eraser may optionally contain up to wt % titanium dioxide. Preferably, the finished eraser composition should have a hardness of from 70 to 85 Shore A. The calcium carbonate preferably should be dry ground and have a mean particle size of from 6 to 20 microns, with 12 microns being more preferred. The preferred pumice range should be from 2 to 3 wt %
A preferred composition is comprised of 22-41 wt % styrene-ethylene/butylene-styrene block copolymer, 7-9 wt % ethylene/propylene copolymer, 30-50 wt % calcium carbonate, 17-19 wt % factice, 0.1-7 wt % pumice, and 0.1-1 wt % titanium dioxide.